Conventionally, although plastic materials derived from petroleum, which is a limited resource, are being used in many occasions, techniques involving less burdens to the environment (environmental friendliness) are given high profiles in the recent years. Under the background art mentioned above, materials using cellulose fibers which are biomass that is present in an abundant amount in nature are currently remarked.
For example, Non-Patent Publications 1 to 3 describe that a composite material of a polylactic acid and cellulose nanowhiskers is obtained by using a dispersion prepared by dispersing acicular fine cellulose fibers called cellulose nanowhiskers in an organic solvent such as toluene, cyclohexane, or chloroform. The cellulose nanowhiskers are obtained by hydrolyzing a raw material cellulose with sulfuric acid, and subjecting the hydrolyzed mixture to a ultrasonic treatment, but the cellulose nanowhiskers by themselves have some disadvantages in dispersibility in an organic solvent such as an alcohol or a non-aqueous solvent. Therefore, in order to prepare a dispersion, in the Non-Patent Publications 1 to 3, the cellulose nanowhiskers are treated with an anionic surfactant such as a phenyl group-containing phosphoric ester to modify (to hydrophobically treat) surfaces thereof, thereby making it possible to stably disperse the cellulose nanowhiskers in an organic solvent.
In addition, the present applicant previously reports that a composite material having high elastic modulus, tensile strength, and transparency is obtained by using cellulose fibers having an average fiber diameter of 200 nm or less and a carboxyl group content of from 0.1 to 2 mmol/g as fine cellulose fibers having a nanosize fiber diameter, and mixing a polylactic acid with the cellulose fibers, cf. Patent Publication 1. The fine cellulose fibers are obtained by subjecting natural cellulose fibers such as wood pulp to an oxidation treatment in the presence of a 2,2,6,6-tetramethyl-1-piperidinyl-N-oxyl (TEMPO) catalyst, and subjecting the resulting dispersion of an oxidized product to a disintegration treatment with a mixer or the like to form microfibrils, and the fine cellulose fibers have even finer fiber diameters than the fibers conventionally called nanofibers.
Further, since the above-mentioned fine cellulose fibers do not have sufficient dispersion stability in an organic solvent or a resin, the present applicant prepares a fine cellulose fiber composite by adsorbing a surfactant containing a cationic surfactant such as a primary to tertiary amine compound, or a quaternary ammonium compound to the fine cellulose fibers. Since the fine cellulose fiber composite has excellent dispersion stability in an organic solvent or a resin, it is suitable for forming composites with a plastic material such as a polylactic acid, thereby making it possible to provide an environmental-friendly composite material having both high mechanical strength and transparency, cf. Patent Publication 2.
On the other hand, Non-Patent Publication 4 discloses that fine cellulose fibers obtained by an oxidation treatment in the presence of TEMPO catalyst are surface-treated with octadecylamine (ODA) to provide a hydrophobically treated derivative. The binding amount of ODA of the derivative is calculated as 1.58 mmol/g, from the measurement results of conductometric titration. In addition, Non-Patent Publication 5 discloses a derivative obtained by subjecting the fine cellulose fibers obtained by the same TEMPO treatment as above, to a treatment with hexylamine (C6) or dodecylamine (C12). The binding amount thereof is calculated as 0.18 mmol/g for the C6 derivative, and 0.06 mmol/g for C12 derivative, from the measurement results of conductometric titration.